Microelectromechanical system or “MEMS” devices are generally well-know. In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and electronics integrated in the same environment, i.e., on a silicon chip. MEMS is an enabling technology in the field of solid-state transducers, i.e., sensors and actuators. The microfabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks but in combination can accomplish complicated functions. Current applications include accelerometers, pressure, chemical and flow sensors, micro-optics, optical scanners, and fluid pumps. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove unmasked portions thereof The resulting three-dimensional silicon structure functions as a miniature mechanical force sensing device, such as an accelerometer that includes a proof mass suspended by a flexure.
The current state of the art of MEMS accelerometer devices is illustrated, for example, by U.S. Pat. No. 6,294,400 and U.S. Pat. No. 6,308,569. As is generally well-known, current MEMS accelerometer devices rely on architectures which compromise “best practice” design principles in order to obtain a structure that can be fabricated within the design rules of a given architecture. However, architectures of current MEMS accelerometer and other devices limit the level of performance achievable.
Some limitations typical of prior art devices include the mechanical die-stack containing multiple different materials having different coefficients of thermal expansion, some with poor thermal conductivity. Typically, the die-stack that forms the mechanical portion of current state of the art accelerometer devices contains multiple materials. The different materials each have different coefficients of thermal expansion, which results in differential expansion between the different materials over temperature. This differential expansion causes a temperature sensitivity in the accelerometer raw output data which requires temperature compensation be performed, either within the accelerometer or at an upper system level, if high performance is desired in the device. This differential expansion also causes high residual stresses to be present in the different joined materials near the interface. These high residual stresses may appear because the joining process is accomplished at an elevated temperature. Alternatively, the high residual stresses are caused by differential expansion over the device's operating temperature range. In either case, even supposedly “elastic” materials like silicon and silicon-dioxide are visco-elastic at the micro level. Therefore, the residual stress drives dimensional instability which, over time, shows as an instability in the device output. For example, current state of the art of MEMS accelerometer devices illustrated in U.S. Pat. No. 6,294,400 and U.S. Pat. No. 6,308,569 provide an oxide layer that is in direct contact with proof mass anchors or other critical mechanical features. This oxide layer represents an unstable dimensional condition that is free to act directly on the proof mass.
Another limitation typical of prior art devices is that one or more of the multiple different materials in MEMS accelerometer devices have poor thermal conductivity. Borosilicate glass substrates, for example, are often used as the plate to which the proof mass is anchored. Borosilicate glass has very poor thermal conductivity, relative to silicon. Therefore, a relatively large mechanical distortion is generated in the borosilicate glass substrate when subjected to a thermal gradient, as occurs during a transient thermal condition. This relatively large mechanical distortion drives accelerometer errors which cannot be easily compensated due to the time dependency of the transient.
Also, many prior art devices lack symmetry. For example, a proof mass is often anchored to a single substrate plate, without provision of a mirror-image counter plate on the opposite side of the device. When mounted to a chassis at the next assembly level, the nonsymmetrical device is subject to thermal and mechanical forces and moments which produce warping or “dishing” of the mechanism. The resultant warping or dishing of the mechanism drives many accelerometer errors.
Typical state of the art MEMS accelerometer devices are not hermetically sealed at the wafer level. Rather, most MEMS devices are sealed at a higher assembly level, such as in a lead-less chip carrier (LCC). This failure to seal the device at the wafer level permits contamination to present a significant reliability issue, and an additional layer of packaging and assembly is necessary to ensure a contamination-free device.
Typical state of the art MEMS accelerometer devices do not include strain relief between the actuator and/or sensor die-stack and chassis to which it's mounted. Direct rigid joining of the die-stack to the next level of assembly results in distortion of the die-stack due to forces and moments that the chassis imparts to the die-stack.
Another limitation of typical state of the art MEMS accelerometer devices is that the die-stack structure of many MEMS accelerometer devices is not very stiff. The compliant die-stack allows forces and moments imparted by outside sources to cause errors in the accelerometer output data.
Still another limitation of typical state of the art MEMS accelerometer devices is that most current MEMS accelerometer devices devote an area of the die solely for transferring electrical signals into and out of the actuator and/or sensor. Because overall die space is often limited due to either cost or size restraints, allocation of die area solely for transferring electrical signals causes the physical accelerometer proof mass size to be compromised. The resultant smaller proof mass is a direct cause of lowered performance.
Therefore, devices and methods for overcoming these and other limitations of typical state of the art MEMS accelerometer and other devices are desirable.